This invention concerns air bag safety systems for passenger vehicles and more particularly the formation of a deployment opening which allows the air bag to be inflated and to enter the passenger compartment of the vehicle.
In many designs, a deployment opening is formed by a hinged door or doors fit into an opening formed in the trim structure behind which the air bag is installed. Upon initiation of the air bag system, the air bag is rapidly inflated forcing the deployment door to hinge open and allow the air bag to deploy into the passenger compartment.
An example of this is shown in U.S. Pat. No. 4,893,833 issued on Jan. 16, 1990, for a "Closure for an Air Bag Deployment Opening". In that design, frangible shearing tabs are relied on to secure the deployment door against opening which frangible tabs is sheared off upon the inflation of the air bag exerting pressure on the inside of the deployment door.
For another example see also U.S. Pat. No. 4,989,896 issued on Jan. 5, 1991, for a "Double Door Closure for an Air Bag Deployment Opening".
In such designs the deployment door is readily visible to the vehicle occupants. For improved styling and to not invite tampering, there has been developed an invisible seam deployment door installation in which the deployment door and the outline of the opening to be formed are not visible but rather the area whereat the deployment door will be formed when the air bag system is operated is overlain by a smooth covering as of a vinyl covering layer.
In this instance, it is necessary to cut through the covering layer upon initiation of the air bag system operation.
A decorative covering layer of vinyl is relatively tough and difficult to sever. It is also critical to proper operation that the air bag deployment occur within a very brief time span, on the order of several milliseconds.
Significant delay in the deployment of the air bag can defeat or seriously impair the effectiveness of the air bag system.
In U.S. Pat. No. 5,217,244 issued on Jun. 8, 1993, for an "Arrangement for Forming and Air Bag Deployment Opening", and U.S. Pat. No. 5,080,393 issued on Jan. 14, 1992, for a "Method and Apparatus for Forming an Air Bag Deployment Opening" there is described the use of linear energy generating elements to completely or partially degrade the vinyl in a pattern overlying the deployment door or doors such as to ensure instantaneous severing of the covering layer.
U.S. Pat. No. 5,082,310 issued on Jan. 21, 1992, for an "Arrangement for Providing and Air Bag Deployment Opening" describes an arrangement for preweakening of the covering layer and exerting an increased pressure by the deployment doors such as to ensure severing of the covering layer.
For both the double door installations the severing of the covering layer depends in large measure on the shearing effect created by the stationary covering layer bonded to the adjacent instrument panel structure and the moving section of covering layer attached to the deployment door.
The shearing of the entire perimeter of the single door installation is particularly critical inasmuch as a single door design requires a greater extent of hinging movement of the deployment door in order to allow the air bag to pass through the deployment opening.
The present inventors have discovered that the adjacent structure of the trim piece, i.e., the instrument panel defining the opening into which the deployment door is fit, may distort appreciably during the air bag deployment and that this distortion will substantially affect the shear severing of the covering layer around the perimeter of the deployment door.
A substrate panel defining the deployment door and the adjacent instrument panel substrate are joined by the overlying covering layer which extends over both in the invisible seam design. An intermediate foam layer is typically included. The foam layer is bonded to both the covering layer and the deployment door substrate panel and the instrument panel substrate.
Thus, as the deployment door substrate panel attempts to hinge open, a force is exerted on the instrument panel structure tending to bulge the instrument panel structure outwardly.
This outward bulging creates several effects. Firstly, the bulging of the instrument panel itself to a significant degree delays the development of sufficient shear forces to sever the covering layer movement of the instrument panel with the deployment door relieves shear stress and movement.
Secondly, the instrument panel bulging, combined with the bulging of the deployment door from its center tends to convert the shear stress into a tensile stress of the covering layer. The covering layer, usually constructed of a plastic material such as vinyl, is able to elongate to a great extent under tension prior to failure particularly of elevated ambient temperatures.
Accordingly, a stretching tendency results in a substantial further delay in completing the severing of the covering layer.
There also results a mispositioning of the edge of the deployment door and the edge of the instrument panel defining the deployment opening. The vinyl covering layer is preferably preweakened by a grooving or cutting of the inside of the covering layer and the system is designed for the covering layer to sever along the grooving in a defined pattern matching the deployment door. The mispositioning described tends to shift the maximum stress exerted on the covering layer from the preweakening pattern.
Another problem associated with excessive deformation of the trim piece defining the deployment opening is that it may be permanently deformed thereby such as to require replacement in the event of an air bag system activation even though the collision triggering the activation did not produce enough vehicle damage to otherwise destroy the trim structure. Accordingly, the replacement of the air bag after activation will also require replacement of the entire instrument panel, substantially increasing the cost of repair.
Accordingly, it is the object of the present invention to provide a construction in which the air bag pressure on the deployment door or doors rapidly and reliably generates a shear stress on the covering layer sufficient to cause complete severing thereof and enable the deployment door to hinge open as quickly as possible to allow proper deployment of the air bag within the passenger compartment.
It is another object of the present invention to stabilize and reinforce the trim piece structure such as to avoid the destruction of the trim piece by activation of the air bag system.